1. Field of the Invention
The present disclosure relates generally to a scintillation detector system including a scintillator and photomultiplier tube, and more specifically to a process for automatically controlling gain of a scintillation detector system (SDS).
2. Background Art
A photomultiplier tube (PMT) is a sensitive detecting device used to measure light or convert light into amplified electrical signals. A typical PMT includes an evacuated glass tube and a series of electrodes disposed within the tube. The series of electrodes includes a photocathode from which a light source enters the tube, a focusing electrode, a plurality of dynodes that function as an electron multiplier, and an anode where the multiplied charge accumulates.
When incident photons (incident light) strike the photocathode of the PMT, the photons eject photoelectrons due to the photoelectric effect. The photoelectrons emitted from the photocathode are accelerated by an electric field, and are directed toward the electron multiplier by the focusing electrode. The electron multiplier, i.e., the series of dynodes, multiplies the photoelectrons by process of secondary emission. When the multiplied photoelectrons reach the anode, they are output as an electrical signal.
More specifically, when the accelerated photoelectrons strike the first dynode, secondary electrons are emitted through secondary emission. These secondary electrons join the first batch of photoelectrons and are accelerated toward the next dynode. This process is repeated over successive dynodes. This cascade effect of secondary emission results in an increasing number of electrons produced at each successive dynode. In other words, charge is amplified at each successive dynode. When the electrons reach the anode, they are output as an amplified electrical signal. As a result of the above process, even a small photoelectric current from the photocathode can provide a large output current at the anode of the PMT. The amplification depends on the number of dynodes, accelerating voltage, temperature, etc.
PMTs are commonly used in scintillation counters (or scintillation detectors) to measure ionizing radiation given off by radiation sources. A scintillation counter is constructed by coupling a PMT to a scintillator. The scintillator produces light when excited by ionizing radiation. The PMT detects and absorbs the light emitted by the scintillator, and, through the process described above, remits the light in the form of amplified electrical output pulses. These pulses may be counted by, e.g., an electronic counter. By analyzing these output pulses, a pulse distribution or energy spectrum may be obtained. Distinct peaks at each energy level can be evaluated as pulse height resolution.
PMTs provide advantages such as high internal gain, high sensitivity, fast responses, low noise, and a high frequency response. However, stability of the gain in the output signal of the PMT may fluctuate due to various factors such as temperature, aging rate of the PMT or scintillator, fluctuations in power supply, material of the photocathode, etc.